A sea spider (Achelia echinata)

Distribution data supplied by the Ocean Biodiversity Information System (OBIS). To interrogate UK data visit the NBN Atlas.Map Help



Achelia echinata has a small, flat body up to 0.15-0.2 cm in length bearing a head, a pair of palps, a pair of ovigers and four pairs of legs. The body is oval in shape and divided into four segments that, although ill-defined, are marked on the dorsal surface by a suture between each of the first three segments. The surface of the body is covered with spines that are divided into two branches each bearing smaller branches. The body is coloured yellowish-brown to cream. The head is fused with the first body segment and bears an ocular tubercule, a proboscis, a pair of chelifores and a mouth that is situated within three pointed lips. The ocular tubercule is slightly conical in shape, has a sharp spine at the tip and bears two pairs of eyes, one pair facing forward the other facing backwards. The proboscis is about the same length as the body and is tapered towards the tip. The chelifores are about half the length of the body and bear small, squat chelae. The palps protrude from the first segment and are divided into eight segments. The ovigers extend from the ventral surface of the first segment and are divided into nine segments. The legs extend from lateral processes of each body segment and are approximately three times the length of the body. Each leg terminates with a claw and is divided into eight segments each bearing a number of spines that characteristically consist of one pair on on the lateral process and two pairs on the first segment.

Recorded distribution in Britain and Ireland

Achelia echinata is the most common British species of Achelia and is widely distributed on the coasts of Britain and Ireland.

Global distribution

Achelia echinata is distributed throughout the North East Atlantic and the Mediterranean.


Achelia echinata inhabits rocky shores from the mid-water zone to depths of 24 m. It is common in rock pools and crevices and amongst various types of algae, bryozoans, sponges and hydroids.

Depth range

0-160 m

Identifying features

  • Small, flat, oval-shaped body up to 0.2 cm in length.
  • Body divided into four segments that are defined by sutures on the dorsal surface, between the first three segments.
  • Body surface covered with bi-branched spines.
  • Proboscis tapered and same length as body.
  • Chelifores half the length of the body.
  • Legs three times the length of the body bearing one pair of spines on lateral process and two pairs on first segment.
  • Males have a protrusion on second segment of third and fourth pairs of legs.

Additional information

Achelia echinata are a slow-moving species. They are carnivorous, suctorial grazers preying on bryozoans, sea anemones, hydroids such as Dynaema pumila and algae such as Griffithsia and Ulva. This species has been known to feed on Flustra foliacea by lying in wait until the bryozoan opens its operculum then putting its proboscis inside the zooid. Reproduction is via external fertilization and the release of protonymphan larvae.

Listed by

- none -


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  1. Centre for Environmental Data and Recording, 2018. IBIS Project Data. Occurrence dataset: https://www.nmni.com/CEDaR/CEDaR-Centre-for-Environmental-Data-and-Recording.aspx accessed via NBNAtlas.org on 2018-09-25.

  2. Centre for Environmental Data and Recording, 2018. Ulster Museum Marine Surveys of Northern Ireland Coastal Waters. Occurrence dataset https://www.nmni.com/CEDaR/CEDaR-Centre-for-Environmental-Data-and-Recording.aspx accessed via NBNAtlas.org on 2018-09-25.

  3. NBN (National Biodiversity Network) Atlas. Available from: https://www.nbnatlas.org.

  4. OBIS (Ocean Biodiversity Information System),  2024. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. www.iobis.org. Accessed: 2024-06-15


This review can be cited as:

Richards, S. 2008. Achelia echinata A sea spider. In Tyler-Walters H. and Hiscock K. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 15-06-2024]. Available from: https://marlin.ac.uk/species/detail/34

Last Updated: 03/07/2008